Industrial control systems, automotive electronics, medical treatment apparatus, naval vessels, aircraft, and security systems are examples of systems which may employ electronic control and monitoring devices connected in a communication network with sensors and actuator devices. In the relevant art, the control devices are generally referred to as “masters” and the sensor and actuator devices are referred to as “slaves”.
In some systems, the slaves are both powered by and receive communications from a source that can be regarded as a sub-system of the master. The incoming cable to each slave comprises conductors used for communication, the supply voltage, and a return conductor. To reduce the weight, cost and rigidity of the cable harness that connects the master to the slaves, it is possible to use the conductors that carry the signals for the dual purpose of delivering power to and communicating with the slaves. This dual use of a conductor for both power and communication is known as DC Power Line Communication (“PLC”). In PLC, power and data may be sent between PLC nodes using only two conductors, eliminating the need for additional wires to interconnect devices.
Existing PLC systems have been adapted for systems in which cost and size (component footprint) are not a significant consideration. For example, Power-over-Ethernet (PoE) uses two twisted pairs, and a DC-DC converter at each end of the link. The implementation of PoE is therefore relatively expensive and requires too much circuitry for compact slave devices. Another group of systems use current injection for the transmitter and AC coupled voltage detection for the receiver in such manner that they cannot make use of the numerous off-the-shelf transmitters and receiver integrated circuits that exist for standards such as RS-485. Not only does this complicate their implementation but it also means that it is impractical to design a master or slave that can be used with or without power line communication.
Other PLC systems utilize only a cable shield to protect from common mode interference which may be insufficient in many applications. Exposure to common mode interference is a particularly significant problem in applications in which the DC PLC cable is positioned proximate to an EMC aggressor such as the power conductors to a servo motor. One notable system, described in the Sick-Stegmann GmbH publication “Hiperface DSL Interface Manual” attempts to deal with common mode interference through use of a transformer and impedance-matching but has the drawback that it uses a number of transformer, capacitor and inductor components that occupy a large amount of board area and decrease transmission efficiency.
For example, FIG. 3 illustrates a communication system 300, in which a “master” 302 transmits and receives signals over a communication medium 330 to and from a “slave” 303. The master 302 transmits to the communication medium 330 first through an impedance matching network 304 and transformer 305. The transformer 305 includes a primary side inductor 307 and a secondary inductor 308. A signal path from a first terminal of the secondary inductor 308 leads through capacitor 315 and then branches, with one path leading to a single-winding inductor 322 (which decouples the voltage level from the signal) and another leading to the communication medium 330. Similarly, a signal path from a second terminal of the secondary inductor 308 leads through capacitor 316 and then branches, with one path leading to another single-winding inductor 324 and another leading to the communication medium 330. An analogous arrangement of inductors and capacitors is employed on the slave side of the circuit. The separate, single-winding inductors 322 and 324, which are distinct from the transformer 305, and the additional capacitors 315 and 316 occupy an undue amount of circuit board space and increase the overall costs of the communication circuit.
It would therefore be advantageous to provide a system and method for improved DC power line communication that is robust against common mode interference, is efficient in terms of occupying as little board space as possible, and has improved transmission efficiency.